diff --git a/lib/jbuilder/key_formatter.rb b/lib/jbuilder/key_formatter.rb
index e73916b..808e36b 100644
--- a/lib/jbuilder/key_formatter.rb
+++ b/lib/jbuilder/key_formatter.rb
@@ -12,7 +12,7 @@ def initialize(*formats, **formats_with_options)
     end
 
     def format(key)
-      @mutex.synchronize do
+      @cache[key] || @mutex.synchronize do
         @cache[key] ||= begin
           value = key.is_a?(Symbol) ? key.name : key.to_s